S100A9 and neoplasm: As well as contributing to the recruitment of neutrophils and myeloid-derived suppressor cells (MDSCs) into tumours (Ehrchen et al., 2009), the S100A8-S100A9 heterodimer (calprotectin) is synthesized by MDSCs and regulates their functions (Acharyya et al., 2012; Lukanidin and Sleeman, 2012).